In line Prompt Gamma Neutron Activation Analysis (PGNAA) analyzers are in wide use throughout the coal, cement, and minerals industries. These systems are used for measuring bulk material, such as rock material coming out of a mine. They do not just do a surface measurement such as X-ray fluorescence and X-ray diffraction, but the analysis is deeply penetrating, and can thus analyze large quantities of materials. The most prevalent type of PGNAA analyzer is an on-belt conveyor analyzer, where all of the material on the conveyor belt is analyzed.
A commercially successful PGNAA analyzer was a chute-type of analyzer, as shown in FIG. 1 and described in 1986 U.S. Pat. No. 4,582,992 to Atwell et al. titled “Self Contained, On-line, Real-Time Bulk Material Analyzer.” Coal or rock produce was sent down the chute, and the material passing through the system was analyzed by the system. U.S. Pat. No. 4,582,992 describes that the PGNAA system was self-contained. These chute systems were very expensive and installation was very costly and difficult. This problem was solved with the development of on-line conveyor-belt PGNAA analyzers. One cross-belt analyzer is shown in FIGS. 2A and 2B, and described in 1995 U.S. Pat. No. 5,396,071 to Atwell et al. titled “Modularized Assembly for Bulk Material Analyzer”. These cross-belt systems were significantly easier to install, and fit very well into the factory operations.
Since the first cross belt was developed, there have been a number of innovations to these cross-belt systems. The innovations have mainly focused on making the system easier to install and manufacture. For example, in U.S. Pat. No. 5,396,071 the belt analyzer was built in multiple identical segments. Segments on the bottom were made from the same mould, and segments on the top were made from a different mould. The central mould was modified to hold the source and detector. Thus this innovation focused on making it easier to build and assemble the analyzer. In Dec. 5, 2000 U.S. Pat. No. 6,157,034 to Griebel et al. titled “Flexible multiple-purpose modular assembly for a family of PGNAA bulk material analyzers,” side modules are used on the conveyor belt analyzer such that the analyzer can be easily configured for different sizes of conveyor belts. This innovation again made it easier for installation and adjustment for different belt sizes, and it simplified manufacturing. In 2002, U.S. Pat. No. 6,657,189 to Atwell et al. titled “Maintaining Measurement Accuracy with Prompt Gamma Neutron Activation Analysis with Variable Material Flow Rates or Material Bed Depths,” the system was designed to algorithmically correct for errors as a result of bed depth and flow rates. This patent was focused on reducing the error that varying flow rates and belt loading can cause in the PGNAA measurement. In W.O. Patent Application No. 2003056317 to Edwards et al. titled “Bulk Material Analyzer and Method of Assembly” discloses a system consisting of detectors and source into a C shape such that the system can slide from the side onto the conveyor belt, and then the other side is added. The main purpose of this design was for ease of installation, and also for simpler manufacturing of the analyzer.
In W.O. Patent Application No. 2008/021228 A3 to Atwell et al. titled “Bulk Material Assembly Including Structural Beams Containing Radiation shielding Material” focuses on making the system easier and less expensive to build, assemble and install. This patent application describes using structural beams that are filled with shielding material to make it faster and easier to install the analyzer, and also reduce the system cost. Thus the design benefit was for easier installation and reduced costs.
Aug. 31, 2010 U.S. Pat. No. 7,786,439 to Harris et al. titled “Detector Apparatus,” discloses the idea of putting the multi-channel analyzer and the detector in a housing that includes a temperature controlled assembly. U.S. Pat. No. 7,778,783 to Lingren et al. titled “Method and apparatus for analysis of elements in bulk substance” discloses a method of stabilizing the spectra coming from the PGNAA analyzer.
Since the development of the first PGNAA on-belt analyzers, the designs have evolved, mainly with the focus of ease of installation and ease of manufacture. Modern PGNAA devices typically mount to the rails of a conveyor belt, do not require cutting of the conveyor belt, and can be installed and calibrated in a few days.
The performance of PGNAA analyzers has not improved dramatically over the 25 years since the systems were first commercialized, as the systems deliver adequate performance for process control for most applications.
The industry with the widest adoption of PGNAA is the cement industry, where the equipment is used to monitor and control the raw material used to make cement.
In the cement industry, there is growing demand for reducing energy costs by increasing the use of alternative fuels. Alternative fuels are materials that can be burned in the cement kiln to provide heat content, and is a replacement for coal and oil. Alternative fuels are byproducts from industrial or commercial operations and include paint, metal cleaning fluids, electronic industry solvents, tires, fly ash, rice hulls, plastics and other industrial or municipal waste. Typically cement plants can obtain these items at little or no cost, or in some cases are paid to burn.
For a cement plant to burn Alternative Fuels (AF), the AF generally includes three characteristics. The first characteristic is that the AF includes little to no elements that negatively impact the cement manufacturing process. For example, alternative fuels with too high a level of chlorine are generally unacceptable for cement plant operations. The Chlorine can turn into hydrochloric acid, and cause erosion in the kiln. Thus each AF end user has an upper limit specification on the amount of Chlorine. The second characteristic is that the AF must have a meaningful heat value, such that it is useful as a fuel. The third characteristic is that the material of the AF complies with environmental regulations. The U.S. regulations essentially state that to be acceptable, the AF cannot have contaminant levels greater than coal. This means that for AF to be acceptable for cement plants, elements such as mercury, arsenic, cadmium, lead, and other hazardous elements must be at a level that is at or below the level of coal.
Currently it is very expensive and time consuming in order to test and qualify new types of alternative fuels. There are companies that specialize in blending alternative fuel for cement plants. The vast majority of these get a very specific and consistent type of feedstock, and they do not work with multiple different materials. Only a very few companies blend varying stock of AF because of the difficulties and challenges in ensuring that the material is suitable for plant operations.
Another factor that makes this issue particularly challenging is that testing AF for trace elements requires very low detection levels that may not possible with conventional PGNAA systems. Thus for receiving a wide variety of alternative fuels, an expensive lab may be required to analyze the AF. A lab can test only a very small sample, and thus may not be a valid way of characterizing the AF.